The Dark Phlogus rests in a shallow pool of Dark Water. When prey approaches, the creature will open along its equator and release an expanding, circular wave of poisonous fluid. While open, it can be shot with any weapon; this will cause the Phlogus to flip over and generate a Safe Zone. It can then be used as a platform until it rights itself a few seconds later. Alternatively, the Phlogus can be destroyed with a Power Bomb.

Interestingly, there is no known Aetherian version of a Dark Phlogus. Evidence suggests, however, that a light Phlogus was scrapped from the game by Retro Studios. The name "Phlogus" appears in scan data lists, and there are unfinished models of a Phlogus in the game's files. This may indicate that a light Aether Phlogus was planned but scrapped.

The Phlogus waits patiently as it floats in murky waters, searching for a meal. When the seismic sensors in its epidermis are triggered, it springs open rapidly to attack. Phlogi can fire a spray of venom to immobilize their prey. A central nervous system is exposed when it opens its mouth to attack: damage this to stun the Phlogus for a time.

Phlogi share a very similar morphology with the Puddle Spore and Spore Spawn, as well as an unnamed mechanoid featured in concept art.

The Dark Phlogus, although obviously an organic life form, physically appears extremely mechanical both in-game and in concept art- most notably, it possesses an outer shell made of regular, tiled hexagons and what appears to be a metal plate on its lower surface.